Abstract: Idiopathic Pulmonary Fibrosis (IPF) is a chronic and highly lethal lung disease characterized by excessive scarring of the lung and whose prevalence is increased in older individuals. The most accepted theory in the pathogenesis of IPF is the injury and activation of alveolar epithelial type II cells (AECII) triggering excessive and chronic wound-healing responses, mediated by growth factors such as TGF-b1. Oxidative stress is recognized as key regulator of the tissue repair process, but the role of redox signaling pathways in the mechanistic pathobiology of IPF remains largely unknown. One NADH-dependent redox enzyme that contributes to stress protection and associates with healthspan and aging is Cyb5R3 (Cyb5R3). Cyb5R3 reduces heme iron (Fe3+?Fe2+), a major target of reactive oxygen species. Our preliminary data suggest that IPF lungs have low expression of Cyb5R3. Using a novel mouse model of conditional deficiency of Cyb5R3 in alveolar type II cells developed in our labs, we have found that 1) expression and activity of Cyb5R3 in AECII are critical for protection against lung fibrosis, 2) Cyb5R3 negatively regulates the expression of TGF-b1- dependent profibrotic and senescence genes, and 3) Cyb5R3 inhibition of expression of TGF-b1 target genes is mediated by two pathways, one the heme reduction of sGC and downstream activation of cGMP-PKG signaling pathway, and second, by control of mitochondrial function and activation of cAMP-PKA. These observations have led to the hypothesis that Cyb5R3 deficiency promotes alteration of the AECII mitochondrial function and redox state that enhances the TGF-b signaling via sGC-dependent and ?independent pathways decreasing the resilience against fibrosis: Aim 1. To test the hypothesis that Cyb5R3 in alveolar epithelial cells confers protection against the development of lung fibrosis through suppression of TGF-b signaling. Aim 2. Establish if loss of AECII Cyb5R3 expression confers efficacy to chronic sGC stimulator and/or sGC activator therapy in mice with lung fibrosis. Completion of these aims will enhance our understanding of the role of Cyb5R3 in the wound healing process and age-related mechanisms of resilience to disrepair and fibrosis.